The increasing availability and use of light emitting diodes based on laser or semiconductor light sources emitting in the near infrared (NIR) region of the electromagnetic spectrum has created the need for high quality photographic imaging materials which are sensitive to this region. For example, imaging materials sensitised to the red and NIR may be used to produce hard copy from computer stored graphics data using a scanning laser diode source. The wavelengths of interest are primarily in the region from 650 to 900 nm, particularly 650 to 850 nm.
Due to environmental concerns, the use of conventional silver halide materials which are processed by known `wet` chemistries are being replaced by `dry` processed materials. Wet processing produces waste materials which are becoming more difficult to dispose of, both commercially and environmentally. Dry processed materials do not suffer from these disposal considerations.
Two main classes of dye have proven useful in the spectral sensitisation of photothermographic imaging materials; namely, merocyanine and cyanine dyes. Merocyanine dyes are often used to spectrally sensitise photothermographic materials to the visible part of the electromagnetic spectrum (400 to 650 nm), but extending their absorption into the NIR region has proven difficult. Cyanine dyes have also been used to spectrally sensitise photothermographic materials to visible radiation. Although frequently subject to residual dye stain problems, as well as being inferior in overall performance to merocyanine dyes, cyanine dyes that absorb in the NIR region can be readily prepared. In particular, heptamethine cyanine dyes, in which the nitrogen-containing heterocyclic end groups are linked by a seven-carbon polymethine chain, are generally found to absorb in the near infrared.
Many photothermographic imaging materials incorporate mercury and other heavy metal salts to enable an acceptable Dmin, a useful sensitivity (speed) (initially and after shelf storage of the coated material), as well as to reduce post processing print out in unexposed areas.
Environmental concerns over the use of mercury and its salts and their subsequent disposal has necessitated the preparation of materials which do not incorporate any mercury species. Consequently, photothermographic materials must be constructed which overcome the loss of the aforesaid advantages. Various non-mercury antifoggants have been proposed, mainly involving heterocyclic tribromomethyl compounds. While these have gained a considerable degree of acceptance, there remains a problem in that sensitising dyes commonly used in mercury-containing systems often give poor results in mercury-free systems.
It will be appreciated that a vast literature exists on the subject of photothermographic media, most of which teaches the use of sensitising dyes. In many cases, the structural formulae given for the sensitiser dyes are very broad and may encompass the dyes used in the present invention. However, there is no evidence of any reduction to practice within the scope of the present invention. The structural formulae generally cover cyanine dyes and usually encompass heptamethine chains, although actual examples of such dyes are rarely given. The substituents on the nitrogen atoms of the cyanine system are usually alkyl groups, which may optionally be substituted with a variety of groups, including carboxyl groups. However, no special advantage is taught for carboxyl groups, least of all in combination with a heptamethine chain.
For example, Japanese Patent No. 63-159841 discloses dry silver type photothermographic media suitable for He--Ne or semiconductor laser exposure. Thirty-one separate general formula are given for the sensitising dyes, one of which defines heptamethine cyanine dyes. However, none of the exemplified dyes has a carboxyalkyl substituent on either of the nitrogen atoms of the cyanine system, the majority being sulphoalkyl-substituted and the remainder alkyl-substituted.
Japanese Patent Publication Nos. 60-140335, 63-231437 and 63-259651 disclose photothermographic media of the dry silver type comprising a range of sensitising dyes encompassing those of Formula (I). None of the exemplified dyes is a heptamethine cyanine dye. One example in JP 60-140335 shows carboxyalkyl substitution on a trimethine dye. Sulphoalkyl substituents are apparently preferred.
U.S. Pat. Nos. 4639414, 4740455, 4741966 and 4751175, and Japanese Patent No. 63-15245 disclose photothermographic media comprising silver halide and a reducible silver salt (usually silver benzotriazole) dispersed in a hydrophilic binder, normally gelatin. Although heptamethine cyanine dyes are encompassed by the general formulae given for the sensitisers there is no example of a heptamethine cyanine dye bearing a carboxyalkyl substituent on either of the nitrogen atoms of the cyanine system.
Japanese Patent No. 63-304242 discloses photothermographic media comprising silver halide, a reducing agent, a binder and a dye-precursor that forms or releases a dye on reaction with the oxidised reducing agent. There is no mention of any silver salt other than the silver halide. The silver halide is IR-sensitised and sixty-seven examples of cyanine and merocyanine sensitiser dyes are given, one of which (No. 64) is a bis-carboxyethyl substituted heptamethine cyanine dye. There is no indication of any advantage associated with this particular compound, all the other cyanine dyes being alkyl or sulphoalkyl substituted. These formulations differ from those of the present invention because they do not contain a light-insensitive reducible silver source and they are aqueous-(gelatin)-based.
U.S. Pat. No. 4835096 discloses the use of heptamethine cyanines dyes with a particular substitution pattern on the terminal aromatic rings as sensitisers for dry silver photothermographic media. Possible substituents for the nitrogen atoms of the cyanine system are said to include alkyl groups of up to 5 carbon atoms, although ethyl is the only example given. There is no mention of substituted alkyl groups, or of mercury-free formulations. Japanese Patent Application No. 03-163440 discloses photothermographic media comprising photosensitive silver halide, an oxidation-reduction image forming component consisting of a reducible organic silver salt and a reducer, a binder, and a sensitising dye of defined structure. The sensitising dyes disclosed have a heptamethine cyanine nucleus in which both nitrogen atoms bear a carboxyalkyl substituent of formula --(CH.sub.2).sub.n CO.sub.2 H where n=1-4. The maximum value of n shown in the Examples is 3, with most examples having n=2.
The present invention provides alternative spectral sensitisers which find application in both mercury-containing and more particularly mercury-free photothermographic materials.